The invention relates to casting systems and methods with auxiliary cooling onto a liquidus portion of the casting. In particular, the invention related to clean metal nucleated casting systems and methods with auxiliary and direct cooling onto a liquidus portion of the casting.
Metals, such as iron-(Fe), nickel-(Ni), titanium-(Ti), and cobalt-(Co) based alloys, are often used in turbine component applications, in which fine-grained microstructures, homogeneity, and essentially defect-free compositions are desired. Problems in superalloy castings and ingots are undesirable as the costs associated with superalloy formation are high, and results of these problems, especially in ingots formed into turbine components are undesirable. Conventional systems for producing castings have attempted to reduce the amount of impurities, contaminants, and other constituents, which may produce undesirable consequences in a component made from the casting. However, the processing and refining of relatively large bodies of metal, such as superalloys, is often accompanied by problems in achieving homogeneous, defect-free structure. These problems are believed to be due, at least in part, to the bulky volume of the metal body and the amount and depth of the liquidus metal during the casting and solidification of the ingot.
One such problem that may often arise with respect to superalloys comprises controlling the grain size and other microstructure of the refined metals. Typically, refining processing involves multiple steps, such as sequential heating and melting, forming, cooling, and reheating of the large bodies of metal because the volume of the metal being refined is generally of at least about 5,000 pounds and can be greater than about 35,000 pounds. Further, problems of alloy or ingredient segregation also occur as processing is performed on large bodies of metal. Often, a lengthy and expensive sequence of processing steps is selected to overcome the above-mentioned difficulties, which arise through the use of bulk processing and refining operations of metals.
A known such sequence used in industry, involves vacuum induction melting; followed by electroslag refining (such as disclosed in U.S. Pat. Nos. 5,160,532; 5,310,165; 5,325,906; 5,332,197; 5,348,566; 5,366,206; 5,472,177; 5,480,097; 5,769,151; 5,809,057; and 5,810,066, all of which are assigned to the Assignee of the instant invention); followed, in turn, by vacuum arc refining (VAR) and followed, again in turn, by mechanical working through forging and drawing to achieve a fine microstructure. While the metal produced by such a sequence is highly useful and the metal product itself is quite valuable, the processing is quite expensive and time-consuming. Further, the yield from such a sequence can be low, which results in increased costs. Furthermore, the processing sequence does not ensure defect-free metals, and ultrasonic inspection is generally employed to identify and reject any components that include such defects, which results in further increase in costs.
A conventional electroslag refining process typically uses a refining vessel that contains a slag-refining layer floating on a layer of molten refined metal. An ingot of unrefined metal is generally used as a consumable electrode and is lowered into the vessel to make contact with the molten electroslag layer. An electric current is passed through the slag layer to the ingot and causes surface melting at the interface between the ingot and the slag layer. As the ingot is melted, oxide inclusions or impurities are exposed to the slag and removed at the contact point between the ingot and the slag. Droplets of refined metal are formed, and these droplets pass through the slag and are collected in a pool of molten refined metal beneath the slag. The refined metal may then be formed into a casting or ingot (collectively referred to hereinafter as xe2x80x9ccastingsxe2x80x9d).
The above-discussed electroslag refining and the resultant casting may be dependent on a relationship between the individual process parameters, such as, but not limited to, an intensity of the refining current, specific heat input, and melting rate. This relationship involves undesirable interdependence between the rate of electroslag refining of the metal, metal ingot and casting temperatures, and rate at which a refined molten metal casting is cooled from its liquidus state to its solid state, all of which may result in poor metallurgical structure in the resultant casting.
Further, electroslag refining may not provide for the controlling of an amount and depth of the liquidus portion in a casting. A reduced solidification rate may result in the casting having properties and characteristics that are not desirable. For example, and in no way limiting, the undesirable characteristics may include inhomogeneous microstructure, defects including (but not limited to) impurities, voids and inclusions, segregations, and a porous (non-dense) material resulting from entrapped air due to slow solidification.
Another problem that may be associated with conventional electroslag refining processing comprises the formation of a relatively deep metal pool in an electroslag crucible. A deep melt pool causes a varied degree of ingredient macrosegregation in the metal that leads to a less desirable microstructure, such as a microstructure that is not a fine-grained microstructure, or segregation of the elemental species so as to form an inhomogeneous structure. A subsequent processing operation has been proposed in combination with the electroslag refining process to overcome this deep melt pool problem. This subsequent processing may be vacuum arc remelting (VAR). Vacuum arc remelting is initiated when an ingot is processed by vacuum arc steps to produce a relatively shallow melt pool, whereby an improved microstructure, which may also possess a lower hydrogen content, is produced. Following the vacuum arc refining process, the resulting ingot is then mechanically worked to yield a metal stock having a desirable fine-grained microstructure. Such mechanical working may involve a combination of steps of forging, drawing, and heat treatment. This thermo-mechanical processing requires large, expensive equipment, as well as costly amounts of energy input.
An attempt to provide a desirable casting microstructure has been proposed in U.S. Pat. No. 5,381,847, in which a vertical casting process attempts to control grain microstructure by controlling dendritic growth. The process may be able to provide a useable microstructure for some applications, however, the vertical casting process does not control the source metal contents, including but not limited to impurities, oxides, and other undesirable constituents. The process, as set forth in the patent, does not control the depth or the liquidus portion or provide anything to enhance the solidification rate of the casting, which may adversely impact the casting""s microstructure and characteristics.
Therefore, a need exists to provide a metal casting process that produces a casting with a relatively homogeneous, fine-grained microstructure, in which the process does not rely upon multiple processing steps that controls the depth of the liquidus portion of the casting. Further, a need exists to provide a metal casting system that produces a casting with a relatively homogeneous, oxide-free, fine-grained microstructure. Also, a need exists to provide a metal casting process and system that produces a casting that is essentially free of oxides and/or entrapped air due to slow solidification rates.
An aspect of the invention sets forth a casting system for producing a metal casting. The casting system comprises auxiliary cooling onto a liquidus portion of the casting and can produce a metal casting that comprises a fine-grain, homogeneous microstructure. The microstructure is essentially oxide- and sulfide-free, segregation defect free, and essentially free of voids caused by air entrapped during solidification of the metal from a liquidus state to a solid state. The casting system with auxiliary cooling onto a liquidus portion of the casting can comprise an electroslag refining system; source of liquid metal, such as a nucleated casting system; and at least one cooling system that supplies coolant onto a liquidus portion of the casting. The casting is cooled in a manner sufficient to provide a microstructure that comprises a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free, segregation defect free, and essentially free of voids caused by air entrapped during solidification from a liquidus state to a solid state.
A further aspect of the invention provides a method for forming a metal casting using auxiliary cooling onto a liquidus portion of the casting. The method produces a metal casting that comprises a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free, segregation defect free, and essentially free of voids caused by air entrapped during solidification of the metal from a liquidus state to a solid state. The method comprises forming a source of clean refined metal that has oxides and sulfides refined out by electroslag refining; forming the casting by a nucleated casting process; and cooling a liquidus portion of the casting. The cooling comprises directing coolant onto the liquidus portion of the casting, wherein the step of cooling is sufficient to provide a microstructure that comprises a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free, segregation defect free, and essentially free of voids caused by air entrapped during solidification from a liquidus state to a solid state.